Stents are well known medical devices that have been used for maintaining the patency of a large variety of vessels of the human body. The most frequent use is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than ten years, many stent designs still lack the required flexibility and radial rigidity to provide an optimum clinical result. Another deficiency of open cell stents is that some stent struts members can flare outward (fish scaling) as the stent is advanced through a tight curve.
An open cell stent is defined as a stent that has circumferential sets of strut members with some curved sections (crowns) that are not connected by a longitudinal connecting link to an adjacent circumferential set of strut members. In comparison, a closed cell stent has every curved section of every circumferential set of strut members, except at the distal and proximal ends of the stent, attached to a longitudinal connecting link. A strut member whose curved section is not attached to a longitudinal connecting link is defined as an unconnected strut member.
There are several “open cell” stents that are currently being marketed for the treatment of coronary stenoses. Examples of these are the Tetra stent from Guidant Corporation and the S670 stent from Medtronics, Inc. Each of these stents has a limited number of straight longitudinal connecting links to join adjacent curved sections of adjacent circumferential sets of strut members. These straight longitudinal connecting links can cause outward flaring of the end circumferential sets of strut members as the stent bends around a curve. The interior unconnected strut members also can flare outward when the pre-deployed stent mounted on a balloon is advanced through a curved vessel such as a coronary artery. Any strut that flares outward can engage the vessel wall during stent delivery in a curved vessel thereby preventing the stent from reaching the site that is to be stented.
Most current open cell stents use a multiplicity of circumferential sets of strut members connected by straight longitudinal connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the arterial wall. A single strut member is defined as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current open cell stent designs such as the Tristar stent (Guidant Corp.) these sets of strut members are formed from a single piece of metal having a uniform wall thickness and uniform strut width. Although a stent with uniform width of the strut members will function, if the width is increased to add strength or radiopacity, the sets of strut members will experience increased strain upon expansion. High strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.
Existing highly radiopaque stents such as the Crossflex coil stent (made from 0.005 inch diameter tantalum wire) by Cordis Corp. and the gold plated NIROYAL stent made by the Boston Scientific Co. can obscure the inside of the vessel because of the high radiopacity over the entire length of the stent. The Be stent of Medtronics, Inc., has small gold markers at the ends of the stent but those markers only mark an end point without allowing visualization of the entire end set of strut members. Fischell et al in U.S. Pat. No. 6,086,604 teaches a closed cell stent with the end sets of strut members being gold plated. Such a stent would have ideal radiopacity but could (like the Be stent) exhibit corrosion due to dissimilar metals placed in an electrolytic solution such as the blood. There has also been significant evidence that gold is a poor surface material for stents because it can increase the risk of subacute thrombosis and restenosis.
Fischell et al in U.S. Pat. No. 5,697,971 show in FIG. 7 a stainless steel stent with increased width diagonal sections in all the circumferential sets of strut members. The U.S. Pat. No. 5,697,971 patent does not teach the use of a variable width for diagonal sections as a means to tailor the stent radiopacity. The U.S. Pat. No. 5,697,971 patent also does not teach specific metals and alloys having greater radiopacity than stainless steel, nor does it teach the use of the alternating curved sections and diagonal sections to form the circumferential sets of strut members. Furthermore, the U.S. Pat. No. 5,697,971 does not teach the use of variable width of curved sections that can provide additional radial rigidity and/or variable width diagonal sections to create relatively greater radiopacity for the end set of strut members.